How to tell whether a pool, beach, or lake is safe for swimming and (adequately!) germ-free

It's really hot. We know. But don't let your desperation to cool down short-circuit your better judgment. Not all swimming situations are created equal, and some can get you into downright hot water when it comes to your health. Lakes, pools, and even the ocean can be dangerous places that land you in the emergency room if you aren't careful. So before you pack your beach bag and cooler, review this list of red flags. These six signs are clear warnings that what might look like a day of summer fun could spell big trouble.

1) The attendant seems distracted Pools need a lot of attention, and the manager on duty should be vigilant. Monitoring the chemicals is especially crucial to healthy waters. A pool's chemical levels should be tested at least twice a day. A texting addict or a flirty teenager are just two species of distracted pool attendants who can forget to test and fail to make adjustments to keep swimmers safe. If you're not sure the pool's chlorine level is being carefully monitored, it may be a virtual petri dish of bad bugs like norovirus, which can cause innocent swimmers bouts of vomiting and cramps.

2) Cloudy water If a pool's water isn't sparkling and clear, don't even think about going in. Not only does this condition suggest poor maintenance, lack of visibility itself poses a threat. Earlier this summer, a woman died in a Boston public pool and her body went undiscovered for 2 days—while swimmers and lifeguards used the pool like usual—because the murky water prevented anyone from seeing her. If the water had been clean and clear, she may have been spotted immediately and saved.

3) Algae Lakes can be beautiful, peaceful spots to cool off in, but nature harbors dangers too. Blue-green algae blooms occur often in lakes, and some produce toxic cyanobacteria. The slimy and often smelly film that floats on the lake's surface is a distinct green color, so look carefully for it before diving in. Since you might not have the whole lake in your sight, a good solid sniff is another measure of protection. If there's an off-smell, there may be algae you can't see. And don't let your dog take the plunge either: Pets have died after exposure.

4) Babies Sure, kids are cute, but pre-potty-trained babies don't make good swimming pals. Even trace amounts of fecal matter can harbor the parasite Giardia lamblia. Diaper leakage—which is hard to prevent, especially in the water—can easily contaminate an entire pool. Accidentally ingest some of the germs, and you're looking at a miserable bout of diarrhea and vomiting you could have avoided by sticking to the adult swim area.

5) Signs of riptide Usually debris washes up onto the beach, so if you see a line of seaweed, foam, or other debris pulled in the opposite direction—out into the wide ocean—there could be a riptide offshore. Even a strong swimmer is no match for those dangerous runaway currents. Other telltale signs of a riptide include patches of water that appear a different shade of blue or a visible channel of choppy surf. Just remember that an absence of these signs does not guarantee a riptide-free dip. You should always be careful and swim where there are lifeguards on duty.

6) There's a big crowd The expression "the more the merrier" may apply to many things, but not a public pool. It's a matter of mathematics: Each new body multiplies the germs, viruses, and bacteria getting into the water. More and more swimmers also divide the lifeguard's attention, which equals less safety for all. Use common sense: If the pool seems way too crowded, it probably is. Cool off with some ice cream instead.